Why does Grease Pencil ui not inherit from bpy.types.Panel?
Generic Grease Pencil Panel Classes

The code you have pasted is from properties_grease_pencil_common.py you will notice that none of the "panel" classes, eg GreasePencilDataPanelare registered.  These classes are defined separately and imported, since a lot of grease pencil functionality is generic.

Now if we look at the 3d view space space_view3d.py for example, notice it imports these classes

from .properties_grease_pencil_common import (
    GreasePencilDataPanel,
    GreasePencilPaletteColorPanel,
)


One of the panels defined in this space

class VIEW3D_PT_grease_pencil(GreasePencilDataPanel, Panel):
    bl_space_type = 'VIEW_3D'
    bl_region_type = 'UI'

    # NOTE: this is just a wrapper around the generic GP Panel


which inherits from bpy.types.Panel and is registered, and hence view-able as a panel in the 3D view space.